A clinical isolate of Aeromonas sobria with three chromosomally mediated inducible β-lactamases: a cephalosporinase, a penicillinase and a third enzyme, displaying carbapenemase activity

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Abstract
Hydrolytic profiles of Aeromonas sobria 163a, non-induced and induced, and of a derepressed mutant 163a-M, indicate that this strain has inducible β-lactamases with activities against cephalosporins, penicillins and carbapenems. Three enzymes were identified and two of the β-lactamase genes, ampS, encoding a penicillinase and cepS, encoding a cephalosporinase, were cloned into Escherichia coli, permitting analysis of the individual enzymes. Isoelectric focusing (IEF) analysis using inhibition profiles with EDTA and BRL 42715, confirmed AmpS (pI 7·9) and CepS (7·0) to be serine β-lactamases. A third β-lactamase displaying hydrolytic activity against the carbapenems was inhibited by EDTA. The carbapenemase had a pI of 9·3 and was detected on IEF gels by overlaying the gel with agarose containing imipenem and the chromogenic pH indicator, bromothymol blue. Co-inducibility and the recovery of a derepressed mutant in which all three enzymes were produced at high levels indicate that this isolate ofA. sobria has three co-ordinately controlled β-lactamase genes.

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